FIG. 1 is an explanatory view illustrating a mesh mode communication network. In general, communication can be performed in a broadband wireless access system by using a mesh mode shown in FIG. 1 as well as a point-to-multipoint (hereinafter, referred to as “PMP”) mode. The mesh mode allows access to a base station through relay of another subscriber group in order to actively adapt to the metropolitan indirect wave communication environment where a shadow area exists due to large-scaled buildings.
In the mesh mode, a control sub-frame and a data sub-frame are used instead of existing frames. The control sub-frame comprises a network control sub-frame and a schedule control sub-frame to perform two basic functions. In other words, the network control sub-frame serves to make a connection between different systems and maintain such a connection while the schedule control sub-frame serves to perform equivalent scheduling in data transmission between systems. All the frames other than the network control sub-frame generated periodically are schedule control sub-frames, wherein the length of the control sub-frame is represented by a fixed MSH-CTRL-LEN (network descriptor). The network descriptor which is accompanied with network configuration after network entry allocation during the network control and schedule control sub-frames and indicates distributed scheduling during the schedule control sub-frame is generated within a control frame.
Since IEEE 802.16a standard which is one example of the broadband wireless access system considers indirect wave communication in a band of 2-11 GHz, multi-path fading may seriously occur. In this respect, in order to ensure reliability, an automatic retransmission request (ARQ) scheme is adapted to a medium access control (MAC) layer. Also, an advanced antenna system (AAS) is considered to improve coverage of a cell and system capacity through beam forming based on multi-antenna. A dynamic frequency selection (DFS) function is supported to solve a problem relating to a co-existence with other system in an unlicensed band.
In general, a point-to-multipoint (PMP) mode considered in a broadband wireless metropolitan area network (MAN) and a mesh mode can selectively be supported. The mesh mode allows access to a base station through relay of another subscriber group, and is considered for the metropolitan indirect wave communication environment where a shadow area exists due to large-scaled buildings.
FIG. 2 is an explanatory view illustrating a mesh mode frame structure. As shown in FIG. 2, the mesh mode includes a control sub-frame and a data sub-frame instead of existing frames. The control sub-frame is classified into a network control sub-frame and a schedule control sub-frame to perform two basic functions. In other words, the network control sub-frame serves to make a connection between different systems and maintain such a connection while the schedule control sub-frame serves to perform equivalent scheduling in data transmission between systems. All the frames other than the network control sub-frame generated periodically are schedule control sub-frames. A network descriptor, which constitutes a mesh mode network after network entry allocation during the network control sub-frame and performs distributed scheduling during the schedule control sub-frame, generates schedule control frames. The network descriptor means a central mobile subscriber station that can perform a similar function to that of a base station in the mesh mode.
FIG. 3 is an explanatory view illustrating a concept of a sub-channel in an OFDMA physical layer. Properties of the OFDMA physical layer in a broadband wireless access system will be described with reference to FIG. 3. The OFDMA physical layer divides active carriers into groups and transmits the groups to their respective receiver mobile subscriber stations. The groups of the carriers transmitted to the receiver mobile subscriber stations are referred to as sub-channels. In this case, the carriers constituting the respective sub-channels may be adjacent to one another or may be spaced apart from one another at constant intervals. If multiple access is made for the unit of sub-channel, frequency diversity gain and power concentration gain can be obtained, and forward power control can efficiently be performed.
FIG. 4 is an explanatory view illustrating a resource allocation technique in an OFDMA system. Referring to FIG. 4, slots allocated to respective mobile subscriber stations are defined by a two-dimensional data region, and are a set of successive sub-channels allocated by a burst. As shown in FIG. 4, a data region in OFDMA is schematized by a rectangle determined by two-dimensional combination of a time domain and a frequency (sub-channel) domain. The data region may be allocated to a mobile subscriber station for uplink data transmission, and downlink data can be transmitted to a mobile subscriber station through the data region. To define such a data region in a two-dimensional space, the number of OFDM symbols in the time domain and the number of successive sub-channels in a frequency domain are required, wherein the successive sub-channels start from a position spaced apart by offset from a reference point.
FIGS. 5A and 5B are explanatory views illustrating a sub-channel mapping method in uplink and downlink frames. The allocated sub-channel regions are represented by two-dimensions, and data are mapped from the sub-channel of the first symbol for the allocated two-dimensional sub-channel region. In case of the uplink, the allocation region of the allocated sub-channels are first determined by one-dimension. In other words, duration is determined, and the sub-channels are allocated along a symbol axis from the next of the sub-channel previously allocated to a protocol data unit (PDU) burst. In this case, if it reaches the last symbol of the specific sub-channel domain, it continues to allocate the sub-channels from the next sub-channel.
FIG. 6 is an explanatory view illustrating a frame structure of a communication system using OFDMA. As shown in FIG. 6, one frame includes a downlink (DL) frame and an uplink (UL) frame. The first symbol per frame is used as a preamble, and a mobile subscriber station (MSS) acquires a base station (BS) using the preamble. A downlink map (DL-MAP) and an uplink map (UL-MAP) are medium access control (MAC) messages having information as to how a channel resource is allocated to the uplink and downlink. Also, a downlink channel descriptor (DCD) and an uplink channel descriptor (UCD) are MAC messages indicating physical properties (for example, modulation mode and coding mode) of downlink and uplink channels. The mobile subscriber station and the base station transmit and receive data for the unit of burst using the allocated radio resource in accordance with the uplink map and the downlink map.
FIG. 7 is an explanatory view illustrating a burst allocation scheme. Referring to FIG. 7, two-dimensional blocks for time axis and frequency axis are allocated for a burst in the downlink. In other words, the downlink map includes a start symbol number, a start sub-channel number, the number of used symbols, and the number of used sub-channels. Accordingly, it is noted from the downlink map how the radio resource has been allocated on the frame. Meanwhile, in case of the downlink, the radio resources are sequentially allocated in accordance with a symbol axis corresponding to the first sub-channel and then the radio resources corresponding to the next sub-channel in accordance with the symbol axis are allocated. Accordingly, the uplink map can identify the allocated radio resources through the number of the allocated symbols.
FIG. 8 is a flow chart illustrating network access procedures of a mobile subscriber station in a PMP mode. Referring to FIG. 8, if the power is turned on, the mobile subscriber station scans downlink channels and acquires up/down synchronization with the base station (S41). The mobile subscriber station performs ranging with the base station to adjust an uplink transmission parameter, and is assigned with a basic management connection identifier (CID) and a primary management CID from the base station (S42). The mobile subscriber station performs negotiation with the base station regarding basic performance (S43), and performs authentication procedure (S44). If the mobile subscriber station is registered in the base station, the mobile subscriber station managed by IP is assigned with a secondary management CID from the base station to set IP connection (S45). The mobile subscriber station sets the current date and time (S46), downloads its configuration file from a server (S47), and establishes service connection (S48).
FIG. 9 is a flow chart illustrating a ranging procedure. Referring to FIG. 9, the base station transmits initial ranging information element (IE) having a broadcasting CID by using the downlink map (UL-MAP) message (S51). The mobile subscriber station transmits ranging packets by using a ranging request message (RNG-REQ) in a connection mode state (S52). In the case that the base station receives the ranging packets that cannot be decoded, the base station transmits a ranging response message (RNG-RSP) including a frame number and retry frame information to the mobile subscriber station (S53). If the mobile subscriber station receives the frame number and the retry frame information, the mobile subscriber station adjusts parameters and transmits the ranging request message (RNG-REQ) on the basis of the retry frame information (S54). If the base station receives the ranging packets that can be decoded, the base station transmits a ranging response message (RNG-RSP) including basic management CID (S55). If the mobile subscriber station receives the ranging request message including its MAC address, the mobile subscriber station stores the basic management CID and adjusts other parameters. The base station transmits an initial ranging information element to the mobile subscriber station by using the basic CID of the uplink map message (S56). The base station recognizes its basic CID from the uplink map message, and transmits the ranging request message in response to initial ranging opportunity poll (S57). The base station transmits the ranging response message in response to the ranging request message (S58). The mobile subscriber station which has received the ranging response message adjusts local parameters.
The downlink map (DL-MAP) message defines usage allocated per burst for a downlink duration in a burst mode physical layer while the uplink map (UL-MAP) message defines usage of the burst allocated for an uplink duration.
Table 1 illustrates an example of a downlink map information element.
TABLE 1SyntaxSizeNotesDL-MAP_Message_Format( ) {Management Message Type = 28 bitsPHY Synchronization FieldvariableSee appropriate PHYspecification.DCD Count8 bitsBase Station ID48 bits Begin PHY Specific Section {See applicable PHYsection.for(i= 1; I <= n;i++) {For each DL-MAPelement 1 to n.DL-MAP_IE( )variableSee corresponding PHYspecification.}}if !(byte boundary) {Padding Nibble 4 bitsPadding to reach byteboundary.}}
Table 2 illustrates an example of the uplink map (UL-MAP) message.
TABLE 2SyntaxSizeNotesUL-MAP_IE( ) {CID16 bits UIUC4 bitsif (UIUC == 12) {OFDMA Symbol offset8 bitsSubchannel offset7 bitsNo. OFDMA Symbols7 bitsNo. Subchannels7 bitsRanging Method2 bits0b00 - Initial Ranging/HandoverRanging over two symbols0b01 - Initial Ranging/HandoverRanging over four symbols0b10 - BW Request/PeriodicRanging over one symbol0b11 - BW Request/PeriodicRanging over three symbolsReserved1 bit Shall be set to zero} else if (UIUC == 14) {CDMA_Allocation_IE( )32 bits else if (UIUC == 15) {Extended UIUCVariableSee clauses following 8.4.5.4.3dependent IE} else {Duration10 bits In OFDMA slots (see 8.4.3.1)Repetition coding2 bits0b00 - No repetition codingindication0b01 - Repetition coding of 2used 0b10 - Repetition coding of4 used 0b11 - Repetition codingof 6 used}Padding nibble, if needed4 bitsCompleting to nearest byte,shall be set to 0.}
The information element constituting DL-MAP includes downlink interval usage code (DIUC), a connection ID (CID), and a burst position information (sub-channel offset, a symbol offset, the number of sub-channels, and the number of symbols). A downlink traffic duration corresponding to each mobile subscriber station is divided by the information element. Meanwhile, the information element constituting UL-MAP message defines usage per CID by using uplink interval usage code (UIUC) and determines the position of a corresponding duration by using a ‘duration’ field. In this case, usage per duration is determined by a UIUC value used in the UL-MAP, wherein each of duration starts from a point far away from a previous IE start point by ‘duration’ determined by the UL-MAP IE.
Table 3 illustrates an example of the DL-MAP IE.
TABLE 3SyntaxSizeNotesDL-MAP_IE( ) {DIUC4 bitsif (DIUC == 15) {Extended DIUC dependent IEvariableSee clauses following 8.4.5.3.1} else {if (INC_CID == 1) {The DL-MAP starts with INC_CID = 0.INC_CID is toggled between 0 and 1 bythe CID-SWITCH_IE( ) (8.4.5.3.7)N_CID8 bitsNumber of CIDs assigned for this IEfor (n=0; n< N_CID; n++) {CID16 bits }}OFDMA Symbol offset8 bitsSubchannel offset6 bitsBoosting3 bits000: normal (not boosted); 001:+6 dB; 010: −6 dB; 011: +9 dB; 100:+3 dB; 101: −3 dB; 110: −9 dB; 111: −12 dB;No. OFDMA Symbols7 bitsNo. Subchannels6 bitsRepetition Coding Indication2 bits0b00 - No repetition coding 0b01 -Repetition coding of 2 used 0b10 -Repetition coding of 4 used 0b11 -Repetition coding of 6 used}}
Table 4 illustrates an example of the uplink map information element.
TABLE 4SyntaxSizeNotesUL-MAP_IE( ) {CID16 bits UIUC4 bitsif (UIUC == 12) {OFDMA Symbol offset8 bitsSubchannel offset7 bitsNo. OFDMA Symbols7 bitsNo. Subchannels7 bitsRanging Method2 bits0b00 - Initial Ranging/Handover Ranging overtwo symbols0b01 - Initial Ranging/Handover Rangingover four symbols0b10 - BW Request/Periodic Ranging overone symbol0b11 - BW Request/Periodic Ranging overthree symbolsReserved1 bit Shall be set to zero} else if (UIUC == 14) {CDMA_Allocation_IE( )32 bits else if (UIUC == 15) {Extended UIUCVariableSee clauses following 8.4.5.4.3dependent IE} else {Duration10 bits In OFDMA slots (see 8.4.3.1)Repetition coding2 bits0b00 - No repetition coding 0b01 -indicationRepetition coding of 2 used 0b10 -Repetition coding of 4 used 0b11 -Repetition coding of 6 used}Padding nibble, if needed4 bitsCompleting to nearest byte, shall be set to 0.}
The uplink duration defined by UIUC 12 is allocated for initial ranging, handover ranging, periodical ranging or band request, and has a competition-based characteristic.
As shown in Table 4, the information element constituting the UL-MAP message defines usage per CID by using the uplink interval usage code (UIUC) and determines the position of a corresponding duration by using a ‘duration’ field. In this case, usage per duration is determined by a UIUC value used in the UL-MAP, wherein each of duration starts from a point far away from a previous IE start point by ‘duration’ determined by the UL-MAP IE.
For a mobile communication system including a broadband wireless access system, a relay station has been suggested to improve throughput or eliminate a shadow area, wherein the relay station serves to relay signals between the base station and the mobile subscriber station (MSS). In other words, the relay station serves to transmit the signals from the base station to the mobile subscriber station in case of the downlink while the relay station serves to transmit the signals from the mobile subscriber station to the base station in case of the uplink. The relay station may be fixed to a specific area or may be used as a semi-fixed type. Also, the relay station may be used as a mobile type by being installed in a public transportation means.
The relay station can be used for enlargement of service coverage of the base station and improvement of throughput. The operation of the relay station can depend on its usage.
In the case that the relay station is used for enlargement of service coverage of the base station (Type 1), the relay station relays all the control messages, which are transmitted from the base station or a mobile subscriber station, as well as data transmitted and received between the mobile subscriber station and the base station. In the case that the relay station is used for improvement of throughput (Type 2), the relay station relays user data only exchanged between the mobile subscriber station and the base station, and allows the mobile subscriber station and the base station to directly exchange a broadcasting type control message of the base station or an uplink control message of the mobile subscriber station with each other. The data relayed by the relay station may be delayed in comparison with the case where the mobile subscriber station and the base station directly exchange the data with each other. The relay station provides good signal quality to the mobile subscriber station where data are relayed, and relays the data to the corresponding mobile subscriber station by using a proper channel coding rate and a proper modulation mode, thereby improving total throughput.
However, the mobile communication system provided with the relay station has a problem in that the system fails to suggest how to perform scheduling and allocate a resource between the base station and the relay station and between the relay station and the mobile subscriber station. Also, in the OFDMA based mobile communication system, if relay communication is performed by the relay station, a problem relating to how to allocate a radio resource and how to transmit radio resource allocation information occurs.
In view of the technical aspect, the relay station can be divided into two types. First, the relay station simply amplifies (amplifies only the intensity of signal) a signal received from a transmitting mobile subscriber station and transmits the amplified signal to a receiving mobile subscriber station in an analog mode. In this case, since delay little occurs and the relay station has an amplification function only, it is advantageous in view of cost efficiency. However, a problem occurs in that noise may be amplified when the signal is amplified. Second, the relay station decodes the signal received from the transmitting mobile subscriber station and then encodes the decoded signal to transmit the encoded signal to the receiving mobile subscriber station. In this case, noise can be removed and high throughput can be obtained by a higher data rate coding mode. However, a problem still occurs in that delay may occur during decoding and encoding.
In the frame structure of the aforementioned related art mobile communication system, the mobile subscriber station which receives service from the base station cannot recognize the exact start position of the downlink and uplink regions of the relay station with only information received from the relay station. Accordingly, the mobile subscriber station should synchronize with the relay station per frame through RS-preamble. For example, if the position of the relay station region is changed by the base station, the mobile subscriber station has difficulty in recognizing the relay station region. Even though the mobile subscriber station recognizes the relay station region, error in transmission and reception may occur between the mobile subscriber station and the base station as the mobile subscriber station incorrectly recognizes the relay station region.